1. Field of the Invention
The present invention relates to apparatus for the isotope-selective irradiation and adiabatic expansion of gaseous isotope mixtures to which also supplemental gases of an active or neutral type can be added, with a coherent electromagnetic radiation source tuned to a suitable frequency, preferably of a laser, for enriching one isotope in the isotope mixture.
2. Description of the Prior Art
Various enrichment methods known per se can be carried out with such an apparatus for the isotope specific excitation. Of special current interest for the manufacture of nuclear fuel for nuclear power stations is the enrichment of the uranium isotope 235 to a content of about 3 to 4% versus a natural content of only 0.7% as the starting material. Many of the proposals known to date amount to bringing of uranium isotope or its compound as UF.sub.6 by laser excitation into a more highly excited, i.e. higher--energy state and to enable it thereby to enter into a chemical reaction, preferably with a reaction partner, and to separate the reaction products stemming therefrom from the original gas mixture for instance by physical means. It has been found to be particularly advantageous to cool down the gaseous isotope mixture to be fed-in by adiabatic expansion, desirably to 30-50 K., since in this manner a clear separation of the spectra about the isotope shift, particularly in the Q-branches of the UF.sub.6 occurs. This, in turn, makes possible a selective coverage of only the one isotope compound by means of a laser beam the frequency of which is set accordingly.
For a further explanation of an isotope separation or enrichment method of this type, reference is made to German Published Patent Application P No. 24 47 762, corresponding U.S. patent application, Ser. No. 614,213.
In addition to these chemical reactions made possible by laser excitation, the expansion conditions can be chosen so that condensation occurs in the gas jet. The 235 UF.sub.6 molecules is prevented from such condensation by isotope-specific laser excitation or from adsorption at condensing or already condensed particles. The large mass difference between the particles unchanged in the original gas jet and those produced therein can then be utilized for separating the same by physical means, as is proposed, for instance, in the German applications P No. 26 59 590, corresponding U.S. patent application Ser. No. 862,504 and P No. 28 49 162, corresponding U.S. patent application Ser. No. 089,520.
With both basic methods, adiabatic cooling-down of the gaseous starting materials, for instance UF.sub.6, is necessary. The nozzles required therefor must cause a very strong expansion so that the irradiation of the isotope mixture for the isotope-specific excitation can be performed prior to the condensation of the UF.sub.6. This is achieved by supersonic nozzles which are very narrow at their smallest cross section, then are greatly enlarged and impart to the gas jet the desired form without shock, as fast as possible. A disadvantage is the development of a boundary layer at the walls of the nozzle, since the temperature rise caused by friction and heat inflow reduces the selectivity of the desired process and also adversely influences the pressure recovery in following diffusers.
To diminish these detrimental effects, it has been proposed to suction off the boundary layer; see German Patent Application P No. 28 10 444, corresponding U.S. patent application Ser. No. 018,230, or to thermally insulate or additionally cool the nozzle wall, as is proposed in German patent application P No. 28 05 958. Also connecting many short slit nozzles with interposed neutral gas layers in parallel, as proposed in German patent application P No. 29 23 811, corresponding U.S. patent application Ser. No. 149,720, aims in this direction.
All these proposals require a large expenditure of technical means so that the problem arose to conceive an apparatus for the adiabatic expansion and subsequent irradiation, which is of relatively simple mechanical design and has only a small boundary layer component in the gas flow to be irradiated. At the same time, a high gas throughput should be possible with such an apparatus while simultaneously avoiding stagnant gas layers.